Method for pre-sodiation of negative electrode, pre-sodiated negative electrode, and lithium secondary battery comprising same

ABSTRACT

A method for pre-sodiation of a negative electrode, including the steps of: interposing a separator between a sodium ion-supplying metal sheet and a negative electrode to prepare a simple cell; dipping the simple cell in an electrolyte for pre-sodiation; and electrochemically charging the simple cell dipped in the electrolyte for pre-sodiation to carry out pre-sodiation of the negative electrode wherein the electrochemical charging is carried out while the simple cell is pressurized under a pressure of from 100 kPa to 1,000 kPa. A pre-sodiated negative electrode is also disclosed, including: a current collector; a negative electrode active material layer on at least one surface of the current collector. The negative electrode active material layer includes a negative electrode active material; and a coating layer on the surface of the negative electrode active material layer. The coating layer includes Na-carbonate and Na.

TECHNICAL FIELD

The present disclosure relates to a method for pre-sodiation of anegative electrode, pre-sodiated negative electrode and a lithiumsecondary battery including the same. Particularly, the presentdisclosure relates to a method for pre-sodiation of a negative electrodefor improving the initial efficiency of a lithium secondary battery,pre-sodiated negative electrode and a lithium secondary batteryincluding the same.

The present application claims priority to Korean Patent Application No.10-2019-0110928 filed on Sep. 6, 2019 in the Republic of Korea, thedisclosures of which are incorporated herein by reference.

BACKGROUND ART

Recently, energy storage technology has been given an increasingattention.

Efforts into research and development of electrochemical devices havebeen actualized more and more, as the application of energy storagetechnology has been extended to energy for cellular phones, camcordersand notebook PC and even to energy for electric vehicles. In thiscontext, electrochemical devices have been most spotlighted. Among suchelectrochemical devices, development of rechargeable secondary batterieshas been focused. More recently, active studies have been conductedabout designing a novel electrode and battery in order to improve thecapacity density and specific energy in developing such batteries.

Among the commercially available secondary batteries, lithium secondarybatteries developed in the early 1990's have been spotlighted, sincethey have a higher operating voltage, significantly higher energydensity, longer cycle life and a lower self-discharge rate, as comparedto conventional batteries, such as Ni-MH, Ni—Cd and sulfuric acid-leadbatteries using an aqueous electrolyte.

Since the conventional lithium secondary batteries use alithium-intercalated compound, such as LiCoO₂ or LiMn₂O₄, as a positiveelectrode, such batteries have been manufactured by using a carbonelectrode, to which lithium is not intercalated, as a negativeelectrode. In the case of a carbon electrode, a passivated coating filmis formed on the surface thereof upon the initial charge, and thecoating film interrupts insertion of an organic solvent into a gapbetween carbon lattice layers and inhibits decomposition of the organicsolvent. In this manner, stabilization of a carbon structure andreversibility of a carbon electrode can be improved to allow use of thecarbon electrode as a negative electrode for a lithium secondarybattery.

However, since the formation of such a coating film is an irreversiblereaction, lithium ions are consumed, resulting in a decrease in batterycapacity undesirably. In addition, since the charge/discharge efficiencyof the carbon electrode and positive electrode is not completely 100%,lithium ion consumption occurs, as the cycle number is increased,resulting in the problems of a decrease in electrode capacity anddegradation of cycle life.

Many studies have been conducted to solve the above-mentioned problems,and it has been found that use of a pre-lithiated carbon electrode as anegative electrode can provide a high-capacity lithium secondary batterywith no decrease in capacity, since the coating film formation occurringupon the initial charge is performed in advance. It has been also foundthat cycle life can be improved significantly, since such apre-lithiated carbon electrode compensates for lithium ions consumed asthe cycle number is increased.

Then, active studies have been conducted about pre-lithiation of anegative electrode, such as a carbon electrode. Typically, a method forlithiating a carbonaceous active material through a physicochemicalprocess and manufacturing an electrode, and a method forelectrochemically pre-lithiating a carbon electrode have beenconsidered.

However, there is a need for a novel method substituting for theabove-mentioned pre-lithiation methods, since lithium used for thepre-lithiation methods is expensive.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the problems of the relatedart, and therefore the present disclosure is directed to providing amethod for pre-sodiation of a negative electrode for improving theinitial efficiency of a lithium secondary battery.

The present disclosure is also directed to providing a negativeelectrode obtained by the method for pre-sodiation of a negativeelectrode, and a lithium secondary battery including the same.

Technical Solution

In one aspect of the present disclosure, there is provided a method forpre-sodiation of a negative electrode according to any one of thefollowing embodiments.

According to the first embodiment of the present disclosure, there isprovided a method for pre-sodiation of a negative electrode, includingthe steps of:

-   -   interposing a negative electrode between a sodium (Na)        ion-supplying metal sheet and a separator to prepare a simple        cell;    -   dipping the simple cell in an electrolyte for pre-sodiation; and        electrochemically charging the simple cell dipped in the        electrolyte for pre-sodiation to carry out pre-sodiation of the        negative electrode.

According to the second embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin the first embodiment, wherein the electrolyte for pre-sodiationincludes a sodium salt and a non-aqueous solvent.

According to the third embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin the second embodiment, wherein the sodium salt includes NaCl, NaBr,NaI, NaClO₄, NaBF₄, NaB₁₀Cl₁₀, NaPF₆, NaCF₃SO₃, NaCF₃CO₂, NaAsF₆,NaSbF₆, NaAlCl₄, CH₃SO₃Na, CF₃SO₃Na, (CF₃SO₂)₂NNa, sodium chloroborate,sodium lower aliphatic carboxylate, sodium tetraphenylborate, or amixture of two or more of them.

According to the fourth embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin any one of the first to the third embodiments, wherein theelectrochemical charging is carried out, while the simple cell ispressurized.

According to the fifth embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin any one of the first to the fourth embodiments, wherein theelectrochemical charging is carried out to 3-50% of the charge capacityof the negative electrode (based on Na-ion charge capacity).

According to the sixth embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin any one of the first to the fifth embodiments, wherein theelectrochemical charging is carried out to 10-20% of the charge capacityof the negative electrode (based on Na-ion charge capacity).

According to the seventh embodiment of the present disclosure, there isprovided the method for pre-sodiation of a negative electrode as definedin any one of the first to the sixth embodiments, wherein the sodiumion-supplying metal sheet includes a metal selected from sodium, sodiumalloys and mixtures thereof, alone, or includes a metal having asubstrate attached to one surface thereof for supporting the metal.

In another aspect of the present disclosure, there is provided apre-sodiated negative electrode according to any one of the followingembodiments.

According to the eighth embodiment of the present disclosure, there isprovided a pre-sodiated negative electrode obtained by the method asdefined in any one of the first to the seventh embodiments.

According to the ninth embodiment of the present disclosure, there isprovided a pre-sodiated negative electrode including: a currentcollector; a negative electrode active material layer disposed on atleast one surface of the current collector and including a negativeelectrode active material; and a coating layer formed on the surface ofthe negative electrode active material layer and including Na-carbonateand Na.

In still another aspect of the present disclosure, there is alsoprovided a lithium secondary battery according to the followingembodiment.

According to the tenth embodiment of the present disclosure, there isprovided a lithium secondary battery including the pre-sodiated negativeelectrode as defined in the eighth or the ninth embodiment.

Advantageous Effects

The method for pre-sodiation of a negative electrode using anelectrochemical charging process according to an embodiment of thepresent disclosure includes a preliminary reaction with sodium (Na),while not using lithium applied to the conventional pre-lithiation, sothat the efficiency of a lithium secondary battery may be improved.

In other words, according to an embodiment of the present disclosure,sodium ion cheaper than lithium is used to carry out pre-sodiation sothat an irreversible side reaction may be performed preliminarily on thesurface of a negative electrode. As a result, when a lithium secondarybattery is manufactured by using the pre-sodiated negative electrode, itis possible to improve the initial efficiency of the battery duringcharging/discharging.

BEST MODE

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail. Prior to the description, it should be understoodthat the terms used in the specification and the appended claims shouldnot be construed as limited to general and dictionary meanings, butinterpreted based on the meanings and concepts corresponding totechnical aspects of the present disclosure on the basis of theprinciple that the inventor is allowed to define terms appropriately forthe best explanation.

In one aspect of the present disclosure, there is provided a method forpre-sodiation of a negative electrode, including the steps of:

-   -   interposing a negative electrode between a sodium (Na)        ion-supplying metal sheet and a separator to prepare a simple        cell;    -   dipping the simple cell in an electrolyte for pre-sodiation; and    -   electrochemically charging the simple cell dipped in the        electrolyte for pre-sodiation to carry out pre-sodiation of the        negative electrode.

Hereinafter, each step will be explained in more detail.

First, a negative electrode is interposed between a sodium ion-supplyingmetal sheet, which is a metal sheet supplying sodium (Na) ions, and aseparator to prepare a simple cell.

The sodium ion-supplying metal sheet functions as a source for supplyingsodium ions doped to the negative electrode, and may include a sodiumion-containing material selected from sodium, sodium alloys and mixturesthereof. The sodium alloys may include Na—Pb, Na—K alloys, or the like.

The sodium ion-supplying metal sheet may include a metal selected fromsodium, sodium alloys and mixtures thereof, alone, or may furtherinclude a metal having a substrate attached to one surface thereof forsupporting the metal. The substrate may include stainless steel (SUS),aluminum, nickel, titanium, baked carbon, copper, or the like.

The sodium ion-supplying metal sheet may have a thickness of 15-300 μm,or 20-100 μm.

The separator is disposed on one surface of the metal opposite to thenegative electrode. The separator may function to prevent the sodiumion-supplying metal sheet and the negative electrode from being indirect contact with each other. This is because direct contact betweenthe sodium ion-supplying metal sheet and the negative electrode maycause a doping process (sodiation), thereby making it difficult tocontrol the doping process and interrupting a homogeneous doping processon the negative electrode. In other words, the separator may function tostabilize the doping process of the negative electrode. Herein, theseparator may be any separator used for a conventional secondary batterywith no particular limitation.

According to an embodiment of the present disclosure, the negativeelectrode may include, as a negative electrode active material, acarbonaceous material, silicon-based material (e.g. silicon oxide ofSiO_(x) (0<x<2)), Si, or the like.

The carbonaceous material may be at least one selected from the groupconsisting of crystalline artificial graphite, crystalline naturalgraphite, amorphous hard carbon, low-crystalline soft carbon, carbonblack, acetylene black, Ketjen black, Super P, graphene, and fibrouscarbon, and preferably may be crystalline artificial graphite and/orcrystalline natural graphite.

Besides the above-mentioned materials, particular examples of thenegative electrode active material include a metal composite oxide, suchas Li_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1), Sn_(x)Me_(1-x)Me′_(y)O_(z)(Me: Mn, Fe, Pb, Ge; Me′: Al, B, P, Si, an element of Group 1, Group 2or Group 3 in the Periodic Table, or halogen; 0<x≤1; 1≤y≤3; 1≤z≤8), orthe like; lithium metal; lithium alloy; silicon-based alloy; tin-basedalloy; metal oxide, such as SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃,Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄, Bi₂O₅, or the like; a conductivepolymer, such as polyacetylene; a Li—Co—Ni type material; titaniumoxide; lithium titanium oxide, or the like. Particularly, the negativeelectrode active material may include a carbonaceous material and/or Si.

In general, the negative electrode is obtained by applying negativeelectrode slurry prepared by dispersing a negative electrode activematerial, conductive material and a binder in a dispersion medium to anegative electrode current collector, and carrying out drying. Ifnecessary, the electrode slurry may further include a filler.

The negative electrode current collector generally has a thickness of3-500 μm. The negative electrode current collector is not particularlylimited, as long as it has conductivity, while not causing any chemicalchange in the corresponding battery. Particular examples of the negativeelectrode current collector include copper, stainless steel, aluminum,nickel, titanium, baked carbon, copper or stainless steelsurface-treated with carbon, nickel, titanium, silver, etc.,aluminum-cadmium alloy, or the like. In addition, similarly to thepositive electrode current collector, fine surface irregularities may beformed on the surface of the negative electrode current collector toreinforce the binding force to the negative electrode active material.The negative electrode current collector may be used in various shapes,including a film, sheet, foil, net, porous body, foamed body, non-wovenweb, or the like.

The conductive material is added generally in an amount of 1-50 wt %based on the total weight of the mixture including the negativeelectrode active material. Such a conductive material is notparticularly limited, as long as it has conductivity, while not causingany chemical change in the corresponding battery. Particular examples ofthe conductive material include: graphite, such as natural graphite orartificial graphite; carbon black, such as carbon black, acetyleneblack, Ketjen black, channel black, furnace black, lamp black or thermalblack; conductive fibers, such as carbon fibers or metallic fibers;metal powder, such as fluorocarbon, aluminum or nickel powder;conductive whiskers, such as zinc oxide or potassium titanate;conductive metal oxides, such as titanium oxide; conductive materials,such as a polyphenylene derivative; or the like.

Meanwhile, a graphitic material having elasticity may be used as aconductive material, optionally in combination with the above-mentionedmaterials.

The binder is an ingredient which assists binding of the active materialwith the conductive material and binding to the current collector, andis added generally in an amount of 1-50 wt %, based on the total weightof the mixture including the negative electrode active material.Particular examples of the binder include polyvinylidene fluoride,polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrrolidone,tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrenebutadiene rubber (SBR), fluororubber, various copolymers, or the like.

The dispersion medium may include water, alcohols, such as ethanol,acetone, or the like.

The filler is an ingredient which inhibits swelling of the negativeelectrode and is used optionally. Such a filler is not particularlylimited, as long as it is a fibrous material, while not causing anychemical change in the corresponding battery. Particular examples of thefiller include: olefinic polymers, such as polyethylene orpolypropylene; and fibrous materials, such as glass fibers or carbonfibers.

The negative electrode is allowed to face the sodium ion-supplying metalsheet with the separator interposed therebetween, thereby preparing asimple cell. Herein, the negative electrode may be prepared by cuttingthe negative electrode in such a manner that it may be provided with anon-coated tab portion and a negative electrode material-retainingportion.

According to an embodiment of the present disclosure, pre-sodiation iscarried out in an electrolyte for pre-sodiation. To carry outpre-sodiation, the separator and the sodium ion-supplying metal sheetare stacked successively on the negative electrode active material layerof the negative electrode, and then a simple cell may be prepared, afterjigs (e.g. planar jigs) are disposed above and below the negativeelectrode and the sodium ion-supplying metal sheet.

Next, the simple cell is dipped in the electrolyte for pre-sodiation.

The electrolyte for pre-sodiation may include a sodium salt and anon-aqueous solvent.

The sodium salt may include NaCl, NaBr, NaI, NaClO₄, NaBF₄, NaB₁₀Cl₁₀,NaPF₆, NaCF₃SO₃, NaCF₃CO₂, NaAsF₆, NaSbF₆, NaAlCl₄, CH₃SO₃Na, CF₃SO₃Na,(CF₃SO₂)₂NNa, sodium chloroborate, sodium lower aliphatic carboxylate,sodium tetraphenylborate, or a mixture of two or more of them.

The non-aqueous solvent may be any organic solvent used conventionallyin the art with no particular limitation. Preferably, a high-boilingpoint organic solvent may be used so as to minimize consumption of theelectrolyte for pre-sodiation, caused by evaporation during thepre-sodiation.

The non-aqueous solvent may include a carbonate solvent, ester solvent,or a mixture of two or more of them. Particular examples of thenon-aqueous solvent include, but are not limited to: propylene carbonate(PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethylcarbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide,acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran,N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC),gamma-butyrolactone, ethyl propionate, and methyl propionate, and suchsolvents may be used alone or in combination.

The electrolyte for pre-sodiation may further include an additive.

The additive may include vinylene carbonate, vinyl ethylene carbonate,fluoroethylene carbonate, salicylic acid, LiBF₄, lithiumbis(trifluoromethanesulfonyl)imide (LITFSI), lithium bis(oxalate)borate(LiBOB), lithium difluoro(oxalate)borate (LiODFB), or a mixture of twoor more of them.

According to an embodiment of the present disclosure, the simple cellmay be dipped in the electrolyte for pre-sodiation for 1-30 hours sothat the negative electrode may be wetted with the electrolyte forpre-sodiation. When the dipping time satisfies the above-defined range,the negative electrode active material may be wetted sufficiently withthe electrolyte for pre-sodiation, thereby facilitating the subsequentprocess, pre-sodiation. In addition, it is possible to prevent theproblem caused by an excessively long dipping time, the problemincluding degradation of the durability of the electrode and easydetachment of the active material from the current collector duringprocessing. When the electrolyte for pre-sodiation infiltrateshomogeneously into the negative electrode through the wetting, sodiumions isolated from the sodium ion-supplying metal sheet may be diffusedhomogeneously to the negative electrode, and thus pre-sodiation may becarried out with a homogeneous sodium ion concentration over the wholenegative electrode.

To facilitate dipping/wetting of the simple cell in/with the electrolytefor pre-sodiation, the reactor for carrying out wetting may be convertedinto a vacuum state of less than 760 mmHg. Herein, the electrolyte forpre-sodiation with which the simple cell is wetted may have atemperature of 30-60° C.

Then, the simple cell dipped in the electrolyte for pre-sodiation ischarged electrochemically to carry out pre-sodiation of the negativeelectrode.

According to an embodiment of the present disclosure, the simple cellmay be charged electrochemically by using a charger, after the simplecell is dipped in the electrolyte for pre-sodiation and even the innerpart of the simple cell is wetted sufficiently with the electrolyte forpre-sodiation.

Herein, the extent of electric current during charging may be 0.1-10mA/cm², 0.5-3 mA/cm², or 0.5-2 mA/cm². When the extent of electriccurrent satisfies the above-defined range, there is an advantage in thatsodium ions may react stably and homogeneously with the negativeelectrode.

According to an embodiment of the present disclosure, theelectrochemical charging may be carried out to 3-50%, 5-30%, or 10-20%of the charge capacity of the negative electrode (based on Na-ion chargecapacity). When the electrochemical charging is carried out within theabove-defined range, it is possible to improve the initial efficiencyand cycle characteristics of a battery. It is also possible to preventthe problem of degradation of stability caused by excessiveelectrodeposition of sodium. Herein, the charge capacity of the negativeelectrode may be determined from the theoretical capacity of thenegative electrode active material loaded on the negative electrode, andthe electrochemical charging may be carried out in such a manner thatthe charger may stop charging, after the simple cell is charged to apredetermined percentage (%) of capacity calculated within theabove-defined range.

In addition, the electrochemical charging may be carried out, while thesimple cell is pressurized.

The method for pressurizing the simple cell may be any method known tothose skilled in the art with no particular limitation. For example, asa pressurizing member for the simple cell, a system configured to changethe interval between a pair of planar jigs by using a device may beused.

The pressurizing member, such as jigs, may be made of a material havingno reactivity with an organic electrolyte, and particular examples ofthe material include polyetheretherketone (PEEK).

According to an embodiment of the present disclosure, the pressurizingmember may be mounted to the simple cell, after the simple cell isdipped in and wetted with the electrolyte for pre-sodiation. In avariant, the pressurizing member may be mounted to the simple cell inadvance, before the simple cell is dipped in the electrolyte forpre-sodiation, and then the resultant structure may be dipped in theelectrolyte for pre-sodiation so that the simple cell may be wetted withthe electrolyte for pre-sodiation.

Particularly, when the simple cell is prepared by stacking the separatorand the sodium ion-supplying metal sheet successively on the negativeelectrode active material layer of the negative electrode, and disposingjigs above and below the negative electrode and the sodium ion-supplyingmetal sheet, the simple cell provided with the jigs may be dipped in theelectrolyte for pre-sodiation, and then electrochemical charging may becarried out, while the simple cell is pressurized with the jigs.

In a variant, the simple cell not provided with jigs may be dipped inthe electrolyte for pre-sodiation, and then jigs may be mounted to thesimple cell and electrochemical charging may be carried out, while thesimple cell is pressurized with the jigs.

When applying a predetermined pressure by using a pressurizing member,such as jigs, electrochemical charging may be carried out, while thesodium ion-supplying metal sheet faces the negative electrode activematerial layer of the negative electrode with the separator interposedtherebetween. The sodium ion-supplying metal sheet may be pressurizedagainst the negative electrode active material layer by using thepressurizing member under a pressure of 10-3,000 kPa, 50-2,000 kPa, or100-1,000 kPa. When the pressure satisfies the above-defined range,pre-sodiation may be carried out smoothly and the electrode may beprevented from being damaged physically.

When the simple cell is electrochemically charged under pressurization,it can be charged more homogeneously and stably, as compared to chargingin a non-pressurized state.

Then, the negative electrode may be taken out of the electrolyte forpre-sodiation, washed with an organic solvent, and then dried. Theorganic solvent used for washing may include dimethyl carbonate, diethylcarbonate, ethyl methyl carbonate, or the like. In this manner, it ispossible to dissolve sodium salt sufficiently and to wash the negativeelectrode, while causing no damage upon the negative electrode.

The drying may be carried out by the method generally known to thoseskilled in the art. For example, the negative electrode may be dried ina dry room at 20-40° C. for 10 minutes to 5 hours.

In another aspect of the present disclosure, there is provided apre-sodiated negative electrode obtained by the above-described methodfor pre-sodiation.

In addition, the pre-sodiated negative electrode according to anembodiment of the present disclosure includes: a current collector; anegative electrode active material layer disposed on at least onesurface of the current collector and including a negative electrodeactive material layer; and a coating layer including Na-carbonate and Naand formed on the surface of the negative electrode active materiallayer.

The negative electrode is provided with a coating layer includingNa-carbonate and Na and formed on the surface of the negative electrodeactive material layer. The coating layer corresponds to a passivatedfilm formed by the above-described pre-sodiation. In addition, sinceeven the inner part of the negative electrode is wetted sufficientlywith the electrolyte, the coating layer including Na-carbonate and Namay be formed not only on the surface of the negative electrode activematerial layer but also in the internal pores of the negative electrodeactive material layer.

During the pre-sodiation of the negative electrode, the electrolyte forpre-sodiation is in contact with the negative electrode active materialto cause oxidative-reductive decomposition of the ingredients ofelectrolyte at the interface, and the decomposition products may bedeposited or adsorbed on the interface to form a coating layer as a newinterfacial layer.

The coating layer may include Na reduced and deposited after sodium ionsmigrate toward the negative electrode during the pre-sodiation, andNa-carbonate (Na₂CO₃) produced by the reductive decomposition reactionbetween sodium ions and a carbonate compound as an organic solvent. Inaddition to Na-carbonate and Na, the coating layer may further include(CH₂OCO₂Na)₂, (CH₂CH₂OCO₂Na)₂, NaO(CH₂)₂CO₂(CH₂)₂₀CO₂Na, or the like.

The coating layer interrupts insertion of the organic solvent into thenegative electrode active material layer to inhibit decomposition of theorganic solvent, thereby improving stabilization of the negativeelectrode active material structure and negative electrodereversibility. In other words, the reaction of forming the coating layeris a preliminary reaction of the irreversible region of the negativeelectrode active material. Thus, it is possible to prevent the problemsof consumption of lithium ions during the subsequent battery operationand degradation of battery capacity, thereby improving cycle life.

In still another aspect of the present disclosure, there is provided alithium secondary battery including the above-described pre-sodiatednegative electrode. In other words, an electrode assembly may be formedby using a positive electrode including a positive electrode activematerial, a separator and the pre-sodiated negative electrode, and theelectrode assembly and an electrolyte may be introduced to a batterycasing to provide a lithium secondary battery.

Particular examples of the positive electrode active material mayinclude, but are not limited to: layered compounds such as lithiumcobalt oxide (LiCoO₂) and lithium nickel oxide (LiNiO₂), or thosecompounds substituted with one or more transition metals; lithiummanganese oxides such as those represented by the chemical formula ofLi_(1+x)Mn_(2-x)O₄ (wherein x is 0-0.33), LiMnO₃, LiMn₂O₃ and LiMnO₂;lithium copper oxide (Li₂CuO₂); vanadium oxides such as LiV₃O₈, LiV₃O₄,V₂O₅ or Cu₂V₂O₇; lithium nickel oxides represented by the chemicalformula of LiNi_(1-y)M_(y)O₂ (wherein M is Co, Mn, Al, Cu, Fe, Mg, B orGa, and y is 0.01-0.3); ternary lithium manganese composite oxidesrepresented by the chemical formula of LiMn_(2-y)M_(y)O₂ (wherein M=Co,Ni, Fe, Cr, Zn or Ta, and y=0.01-0.1), or Li₂Mn₃MO₈ (wherein M=Fe, Co,Ni, Cu or Zn); LiMn₂O₄ in which Li is partially substituted with analkaline earth metal ion; disulfide compounds; and Fe₂(MoO₄)₃; ternarylithium transition metal composite oxides, such asLi(Ni_(a)Co_(b)Mn_(c))O₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1); orthe like.

The positive electrode active material may be dispersed in a solventtogether with a binder polymer, conductive material and other additivesto form positive electrode mixture slurry. Then, the positive electrodemixture slurry may be coated on at least one surface of a positiveelectrode current collector, followed by drying and pressing, to form apositive electrode.

Non-limiting examples of the positive electrode current collectorinclude foil made of aluminum, nickel or a combination thereof, or thelike, and non-limiting examples of the negative electrode currentcollector include foil made of copper, gold, nickel, copper alloy or acombination thereof, or the like.

The binder polymer, conductive material and other additives used in thepositive electrode may be the same as or different from those used inthe negative electrode. Reference will be made to the above descriptionabout the binder polymer and conductive material.

The separator is interposed between the positive electrode and thenegative electrode, and an insulating thin film having high ionpermeability and mechanical strength is used as the separator. Theseparator generally has a pore diameter of 0.01-10 μm and a thickness of5-300 μm. The separator may include a porous polymer substrate, such asa porous polymer film substrate or porous polymer non-woven websubstrate, alone, or may further include a porous coating layer disposedon at least one surface of the porous polymer substrate and containinginorganic particles and a binder polymer. The porous polymer filmsubstrate may be a porous polymer film made of polyolefin, such aspolyethylene or polypropylene. In addition to polyolefin, the porouspolymer film substrate may be made of polyester, such as polyethyleneterephthalate, polybutylene terephthalate or polyethylene naphthalene;polyacetal; polyamide; polycarbonate; polyimide; polyetherether ketone;polyether sulfone; polyphenylene oxide; polyphenylene sulfide; or thelike, alone or in combination.

Non-limiting examples of the binder polymer include but are not limitedto: polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloro ethylene, polymethyl methacrylate, polybutylacrylate, polybutyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate,polyethylene oxide, polyarylate, cellulose acetate, cellulose acetatebutyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethylpolyvinylalchol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan,carboxymethyl cellulose, or the like.

According to an embodiment of the present disclosure, the binder polymermay be classified into a dispersant binder polymer also functioning as adispersant, and a non-dispersant binder polymer. The dispersant binderpolymer is a polymer having at least one dispersion-contributingfunctional group in the backbone or sidechain of the polymer, and thedispersion-contributing functional group includes an OH group, CN group,or the like. Particular examples of the dispersant binder polymerinclude cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol,cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethylcellulose, or the like. Particular examples of the non-dispersant binderpolymer include the above-listed binder polymers, except the examples ofthe dispersant binder polymer.

For example, the weight ratio of the inorganic particles to the totalweight of the binder polymer and crosslinked polymer may be 50:50-99:1,particularly 70:30-95:5. When the weight ratio of the inorganicparticles to the total weight of the binder polymer and crosslinkedpolymer satisfies the above-defined range, it is possible to prevent theproblem of a decrease in pore size and porosity of the resultant coatinglayer, caused by an increase in content of the binder polymer and thecrosslinked polymer. It is also possible to solve the problem ofdegradation of peeling resistance of the resultant coating layer, causedby a decrease in content of the binder polymer and the crosslinkedpolymer Non-limiting examples of the inorganic particles includeinorganic particles having a dielectric constant of 5 or more,particularly 10 or more, inorganic particles having lithium iontransportability, or a mixture thereof.

Non-limiting examples of the inorganic particles having a dielectricconstant of 5 or more may include BaTiO₃, Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT), Pb(Mg_(1/3)Nb_(2/3))O₃PbTiO₃(PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂, CeO₂, MgO, NiO, CaO, ZnO, ZrO₂,Y₂O₃, Al₂O₃, SiC, AlO(OH), Al₂O₃—H₂O, or a mixture thereof.

As used herein, the term ‘inorganic particles having lithium iontransportability’ refers to inorganic particles which contain lithiumelements and do not store lithium but transport lithium ions.Non-limiting examples of the inorganic particles having lithium iontransportability may include lithium phosphate (Li₃PO₄), lithiumtitanium phosphate (Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3), lithium aluminumtitanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃, 0<x<2, 0<y<1, 0<z<3),(LiAlTiP)_(x)O_(y)-based glass (1<x<4, 0<y<13), such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅, lithium lanthanum titanate(Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3), lithium germanium thiophosphate(Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5), such asLi_(3.25)Ge_(0.25)P_(0.75)S₄, lithium nitride (Li_(x)N_(y), 0<x<4,0<y<2), such as Li₃N, SiS₂-based glass (Li_(x)Si_(y)S_(z), 0<x<3, 0<y<2,0<z<4), such as Li₃PO₄—Li₂S—SiS₂, and P₂S₅-based glass(Li_(x)P_(y)S_(z), 0<x<3, 0<y<3, 0<z<7), such as LiI—Li₂S—P₂S₅, or amixture thereof.

Although there is no particular limitation in the thickness of theporous coating layer, it may be 1-10 μm, or 1.5-6 μm. Also, the porosityof the porous coating layer is not particularly limited, but it may bepreferably 35-65%.

The electrolyte includes conventional electrolyte ingredients, such asan organic solvent and an electrolyte salt. The electrolyte salt thatmay be used is a salt having a structure of A⁺B⁻, wherein A⁺ includes analkali metal cation such as Li⁺, Na⁺, K⁺ or a mixture thereof, and B⁻includes an anion such as PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻,CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, C(CF₂SO₂)₃ ⁻ or a mixture thereof.Particularly, a lithium salt is preferred. For example, LiClO₄,LiCF₃SO₃, LiPF₆, LiAsF₆, LiN(CF₃SO₂)₂ or a mixture thereof may be used.

The organic solvent used for the electrolyte may include a solventgenerally known to those skilled in the art, such as a cyclic carbonatesolvent containing a halogen substituent or not; a linear carbonatesolvent; an ester solvent, nitrile solvent, phosphate solvent, or amixture thereof. Particular examples of the solvent that may be usedinclude propylene carbonate (PC), ethylene carbonate (EC), diethylcarbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC),dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane,tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate(EMC), gamma-butyrolactone (GBL), flouoroethylene carbonate (FEC),methyl formate, ethyl formate, propyl formate, methyl acetate, ethylacetate, propyl acetate, pentyl acetate, methyl propionate, ethylpropionate, propyl propionate, butyl propionate, or a mixture thereof.

Injection of the electrolyte may be carried out in an adequate stepduring the process for manufacturing a battery depending on themanufacturing process of a final product and properties required for afinal product. In other words, injection of the electrolyte may becarried out before the assemblage of a battery or in the final step ofthe assemblage of a battery.

There is no particular limitation in the appearance or casing of thelithium secondary battery according to an embodiment of the presentdisclosure. For example, the lithium secondary battery may have acylindrical shape using a can, prismatic shape, pouch-like shape or acoin-like shape.

In addition, the lithium secondary battery according to an embodiment ofthe present disclosure may include any conventional lithium secondarybatteries, such as lithium metal secondary batteries, lithium ionsecondary batteries, lithium polymer secondary batteries or lithium ionpolymer secondary batteries.

Examples will be described more fully hereinafter so that the presentdisclosure can be understood with ease. The following examples may,however, be embodied in many different forms and should not be construedas limited to the exemplary embodiments set forth therein. Rather, theseexemplary embodiments are provided so that the present disclosure willbe thorough and complete, and will fully convey the scope of the presentdisclosure to those skilled in the art.

Example 1

<Manufacture of Negative Electrode>

First, 92 parts by weight of hard carbon as a negative electrode activematerial, 3 parts by weight of Denka black as a conductive material, 3.5parts by weight of styrene butadiene rubber (SBR) as a binder and 1.5parts by weight of carboxymethyl cellulose as a binder and thickenerwere added to water as a dispersion medium to prepare negative electrodeslurry.

The negative electrode slurry was coated on one surface of a coppercurrent collector having a thickness of 20 μm and dried at a temperatureof 80° C. Then, the negative electrode mixture layer was pressed byusing a roll press device to a porosity of 28% so that it might satisfythe target thickness. After that, the resultant structure was dried in avacuum oven at 130° C. for 8 hours to obtain a negative electrode.

<Pre-Sodiation Based on Electrochemical Charging>

The negative electrode obtained as described above was cut by using acutter capable of cutting the negative electrode in such a manner thatthe negative electrode active material-retaining portion alone, exceptthe non-coated tab portion, might have a dimension of 34 mm×50 mm. Next,the negative electrode was allowed to face a sodium metal/SUS plate(including a SUS plate attached to one surface of sodium metal) as asodium ion-supplying metal sheet with a separator (polypropylene porouspolymer film) interposed therebetween, thereby providing a simple cell.The simple cell including the negative electrode-separator-sodiummetal/SUS plate stacked successively was wetted with an electrolyte forpre-sodiation, including 1 M NaClO₄ dissolved in a solvent includingethylene carbonate (EC) and ethyl methyl carbonate (EMC) mixed at avolume ratio of 3:7, and 2 wt % of fluoroethylene carbonate (FEC) addedthereto, at 25° C. for 3 hours, and then charged electrochemically byusing a charger, while being pressurized under a pressure of 1000 kPathrough pressurization jigs. In this manner, pre-sodiation of thenegative electrode was carried out. Herein, the electric current was setto an extent of 2 mA/cm² and the simple cell was chargedelectrochemically to 20% of the negative electrode charge capacity(based on Na-ion charge capacity). After completing the pre-sodiation,the negative electrode was washed with dimethyl carbonate (DMC) anddried at room temperature, thereby providing a pre-sodiated negativeelectrode.

<Manufacture of Lithium Secondary Battery>

The pre-sodiated negative electrode obtained as described above was cutinto a coin cell size, a separator (polyolefin porous polymer film) wasinterposed between the negative electrode and lithium metal foil as acounter electrode, and then an electrolyte containing 1 M LiPF₆dissolved in a solvent including ethylene carbonate (EC) and ethylmethyl carbonate (EMC) mixed at a volume ratio of 3:7 was injectedthereto, thereby providing a coin-type half-cell.

Example 2

A pre-sodiated negative electrode and a coin-type half-cell includingthe same were obtained in the same manner as Example 1, except that thesimple cell was charged electrochemically to 15% of the negativeelectrode charge capacity (based on Na-ion charge capacity).

Example 3

A pre-sodiated negative electrode and a coin-type half-cell includingthe same were obtained in the same manner as Example 1, except that thesimple cell was charged electrochemically to 10% of the negativeelectrode charge capacity (based on Na-ion charge capacity).

Comparative Example 1

A negative electrode and a coin-type half-cell including the same wereobtained in the same manner as Example 1, except that no pre-sodiationwas carried out.

Comparative Example 2

<Manufacture of Negative Electrode>

First, 92 parts by weight of hard carbon as a negative electrode activematerial, 3 parts by weight of Denka black as a conductive material, 3.5parts by weight of styrene butadiene rubber (SBR) as a binder and 1.5parts by weight of carboxymethyl cellulose as a binder and thickenerwere added to water as a dispersion medium to prepare negative electrodeslurry.

The negative electrode slurry was coated on one surface of a coppercurrent collector having a thickness of 20 μm and dried at a temperatureof 80° C. Then, the negative electrode mixture layer was pressed byusing a roll press device to a porosity of 28% so that it might satisfythe target thickness. After that, the resultant structure was dried in avacuum oven at 130° C. for 8 hours to obtain a negative electrode.

<Pre-Sodiation>

First, Na-metal powder (50 μm) was applied to the negative electrodeobtained as described above in an amount sufficient for carrying outelectrochemical charging to 20% of the negative electrode chargecapacity (based on Na-ion charge capacity), and then the resultantstructure was allowed to stand in an electrolyte including 1 M NaClO₄dissolved in a solvent including ethylene carbonate (EC) and ethylmethyl carbonate (EMC) mixed at a volume ratio of 3:7, and 2 wt % offluoroethylene carbonate (FEC) added thereto, for 3 hours to carry outpre-sodiation. Then, the negative electrode was washed with DMC anddried at room temperature, thereby providing a pre-sodiated negativeelectrode.

<Manufacture of Lithium Secondary Battery>

The pre-sodiated negative electrode obtained as described above was cutinto a coin cell size, a separator (polyolefin porous polymer film) wasinterposed between the negative electrode and lithium metal foil as acounter electrode, and then an electrolyte containing 1 M LiPF₆dissolved in a solvent including ethylene carbonate (EC) and ethylmethyl carbonate (EMC) mixed at a volume ratio of 3:7 was injectedthereto, thereby providing a coin-type half-cell.

<Initial Reversibility Test>

Each of the coin-type half-cells according to Examples 1-3 andComparative Examples 1 and 2 was tested in terms of charge/dischargereversibility by using an electrochemical charger. During charging,electric current was applied at a current density of 0.1 C-rate to avoltage of 0.005 V (vs. Li/Lit). During discharging, each half-cell wasdischarged at the same current density to a voltage of 1.5 V. Herein,the first cycle efficiency was determined as a ratio of dischargecapacity/charge capacity. The results are shown in the following Table1.

TABLE 1 Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 First cycle 94 90 8780 84 efficiency (%)

Referring to Table 1, in the case of Examples 1-3, the preliminaryreaction in the irreversible region of hard carbon through pre-sodiationbased on electrochemical charging provides an improved initialefficiency during charging/discharging of each lithium secondarybattery. On the contrary, when carrying out no pre-sodiation in the caseof Comparative Example 1, the lithium secondary battery shows a lowinitial efficiency. In addition, when carrying out pre-sodiation byloading sodium metal directly on the negative electrode in the case ofComparative Example 2, sodium metal is not used sufficiently forpre-sodiation but remains as it is, and thus the initial efficiency isnot improved significantly.

The present disclosure has been described in detail. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the disclosure, are given by way ofillustration only, since various changes and modifications within thescope of the disclosure will become apparent to those skilled in the artfrom this detailed description.

1. A method for pre-sodiation of a negative electrode, comprising thesteps of: interposing a separator between a sodium (Na) ion-supplyingmetal sheet and a negative electrode to prepare a simple cell; dippingthe simple cell in an electrolyte for pre-sodiation; andelectrochemically charging the simple cell dipped in the electrolyte forpre-sodiation to carry out pre-sodiation of the negative electrode,wherein the electrochemical charging is carried out while the simplecell is pressurized under a pressure of from 100 kPa to 1,000 kPa. 2.The method for pre-sodiation of the negative electrode according toclaim 1, wherein the electrolyte for pre-sodiation comprises a sodiumsalt and a non-aqueous solvent.
 3. The method for pre-sodiation of thenegative electrode according to claim 2, wherein the sodium saltcomprises at least one of NaCl, NaBr, NaI, NaClO₄, NaBF₄, NaB₁₀Cl₁₀,NaPF₆, NaCF₃SO₃, NaCF₃CO₂, NaAsF₆, NaSbF₆, NaAlCl₄, CH₃SO₃Na, CF₃SO₃Na,(CF₃SO₂)₂NNa, sodium chloroborate, sodium lower aliphatic carboxylate,or sodium tetraphenylborate.
 4. (canceled)
 5. The method forpre-sodiation of the negative electrode according to claim 1, whereinthe electrochemical charging is carried out to 3% to 50% of a chargecapacity of the negative electrode based on Na ion charge capacity. 6.The method for pre-sodiation of the negative electrode according toclaim 1, wherein the electrochemical charging is carried out to 10% to20% of a charge capacity of the negative electrode based on Na ioncharge capacity.
 7. The method for pre-sodiation of the negativeelectrode according to claim 1, wherein the sodium ion-supplying metalsheet comprises at least one metal selected from the group consisting ofsodium, and sodium alloys, and optionally includes a metal having asubstrate attached to one surface to support the metal.
 8. Apre-sodiated negative electrode obtained by the method as defined inclaim
 1. 9. A pre-sodiated negative electrode, comprising: a currentcollector; a negative electrode active material layer on at least onesurface of the current collector, wherein the negative electrode activematerial layer comprises a negative electrode active material; and acoating layer on the surface of the negative electrode active materiallayer, wherein the coating layer comprises Na-carbonate and Na.
 10. Alithium secondary battery comprising the pre-sodiated negative electrodeas defined in claim 9.